FIG. 15 is a system diagram illustrating the basic configuration of a nitrogen generator that distills from air as a raw material through cryogenic separation. This nitrogen generator 100 includes a distillation column 101 in which an upper liquid distributor 102, an upper gas-liquid contactor 103, an intermediate liquid distributor 104, and a lower gas-liquid contactor 105 are disposed in this order from the top. The upper gas-liquid contactor 103 and the lower gas-liquid contactor 105 are typically contactors using structured packing.
In a case of using this nitrogen generator 100 to distill nitrogen gas of 700 kPaG (gauge pressure; the same applies below) as a product, air as a raw material is compressed by an air compressor 106 to 760 kPaG. The heat of compression generated by the compression of the air is removed by an aftercooler 107, so that the compressed air is cooled to 40° C. Then, the carbon dioxide, water, and hydrocarbons contained in the air are removed through adsorption by a pre-treatment unit 108 that alternately uses two adsorbers, so that the air becomes purified air.
The purified air after exiting the pre-treatment unit 108 is introduced into a cold box 110 through a purified air stream 109 and cooled to −165° C., which is near the dew point, by a main heat exchanger 111. The cooled purified air is then introduced into a lower portion of the distillation column 101 through a gas introduction stream 112 as ascending gas in the distillation column 101. Nitrogen gas in an upper portion of the distillation column 101 separated by distillation operations inside the column is drawn to a gas discharge stream 113 at the top of the column. Part of the nitrogen gas branches off into a condensation stream 114 and is introduced into a condenser 115.
Meanwhile, at the bottom of the distillation column 101, oxygen-enriched liquid air is separated by the distillation, drawn into a liquid discharge stream 116, and lowered in pressure to 300 kPaG by a liquid-air pressure reducing valve 117, so that the temperature drops to −180° C. due to the Joule-Thomson effect. This low-temperature liquid air is introduced into the condenser 115 and exchanges heat with the above-mentioned nitrogen gas. Consequently, the nitrogen gas is liquefied and the whole low-temperature liquid air is vaporized into low-temperature air. The liquid nitrogen liquefied at the condenser 115 is introduced into the upper portion of the distillation column 101 through a liquid introduction stream 118 as descending liquid in the distillation column 101.
The low-temperature air vaporized at the condenser 115 is introduced into the main heat exchanger 111 through a low-temperature air stream 119, exchanges heat with the purified air to be heated to −140° C., and is drawn in this intermediate temperature state into a turbine inlet stream 120 from an intermediate portion of the main heat exchanger 111. The low-temperature air in the intermediate temperature state is introduced into an expansion turbine 121, in which the low-temperature air is expanded to 30 kPaG and its temperature is lowered to −170° C. by adiabatic expansion. The low-temperature air lowered in temperature by the expansion turbine 121 is introduced into the main heat exchanger 111 again through a turbine outlet stream. 122 and exchanges heat with the purified air to cool the purified air. Consequently, the low-temperature air is sufficiently warmed to a temperature that is several ° C. lower than the purified air, and then discharged from the cold box 110 through a waste gas stream 123.
Also, the remaining portion of the nitrogen gas discharged into the gas discharge stream 113 from the distillation column 101 is introduced into the main heat exchanger 111. Then, as in the low-temperature air, the remaining portion of the nitrogen gas exchanges heat with the purified air to be sufficiently warmed to a temperature several ° C. lower than the purified air. Thereafter, the remaining portion of the nitrogen gas is discharged from the cold box 110 through a product nitrogen gas stream 124 and collected as a product nitrogen gas. In the case of distilling a product nitrogen gas at a pressure of 700 kPaG as described above, the distillation column 101 is operated at a high pressure of 730 kPaG.
In the distillation column 101, the liquid nitrogen introduced into the distillation column 101 from the condenser 115 through the liquid introduction stream 118 is distributed uniformly in the cross-sectional direction of the packed column 101 by the upper liquid distributor 102 and then flows down toward the upper gas-liquid contactor 103. The descending liquid flowing down from the lower end of the upper gas-liquid contactor 103 is distributed uniformly in the cross-sectional direction of the packed column 101 again by the intermediate liquid distributor 104 and then flows down toward the lower gas-liquid contactor 105. This is done so that the flow rate and composition of the descending liquid flowing down inside the upper gas-liquid contactor 103 and the lower gas-liquid contactor 105 while being in gas-liquid contact with the ascending gas, can be uniform.
Meanwhile, a configuration like a distillation column 131 illustrated in FIG. 16 is sometimes adopted in which a single liquid distributor 133 is disposed above a single gas-liquid contactor 132. However, widely used is a packed column 139 in which a gas-liquid contactor is divided vertically into a plurality of parts, for example, divided vertically into two gas-liquid contactors 135, 136, and an upper liquid distributor 137 and a intermediate liquid distributor 138 are provided respectively above the gas-liquid contactors 135, 136, as illustrated in FIG. 17 (see Patent Literature 1, for example).